THE HISTORY of the "GREEN BULLET"
The Different Models
"The Green Bullet mic, also known as the model 520, was introduced in mid 1949 by Shure Brothers Inc. of Chicago, ILL. It was intended for use as a high quality, moderately priced microphone to be used mainly as a communications, recording, and public address microphone. It became popular with ham radio operators as well as commercial use as a dispatch mic for police, fire, and other commercial dispatch uses, as well as military use. The mic was tailored for good speech response, and was made to withstand temperature extremes, and to be practically moisture proof. It is unaffected by weather extremes and salt spray making an ideal choice for coastal areas. The mic was offered in 1950 with a stand and a handle with a built in "squeeze to talk" handle, which was known as the model 520SL. It was the exact same 520 mic with a built in desk stand and a squeeze to transmit switch. Shure also introduced a medium impedance mic, model 520SLB in 1961, which was meant for use where a long cable was needed between the transmitter and the microphone. The 520 mic without the stand was also available as a medium impedance mic called the 520B in 1952.
IMPEDANCE is a measurement of a microphone's resistance when certain voltages are applied to it and is measured at certain frequencies. It's a rather complicated formula and is difficult for the average person to completely understand. In just about all instances when it comes to people asking someone with a mic for sale what the impedance of the microphone is in ohm's, what they are really asking for is the DC resistance of the microphone's coil, or in the case of the Shure 520 Green Bullet, the DC resistance of the built in transformer winding. Impedance is often refered to as "Z", such as High Z or Low Z. So just to make things clear, throughout this website, when I refer to "impedance", I will be refering to the microphone's element DC resistance as measured in ohm's. For example, when I say an element has an impedance of 1,100 ohm's, I am refering to the DC resistance with no load or voltage applied to it. This is not the real definition of impedance, but it is what just about everyone is asking about when they ask for an impedance reading of a mic or element.
The high impedance model's had an impedance (DC resistance) of about 1200 to 1350 ohms in the first few years of production. The medium impedance models were 150 to 320 ohms, but later on in 1961 became available in a 15 ohm model in which the element model was labeled 99C86. The medium impedance models had an element model # 99E86. Over the first 10 years, the high impedance elements had model #'s that were 99A86, 99B86, 99G86, and 99H86, although the first green bullet mic's were made using the model 99A86, and later on in the early 50's changed to 99B86, and then 99G86 in the late 50's.
These models were all basically built the same way, with the same materials, although they were used in many different types of mics including mics made for other companies such as RCA, Wilcox Gay, Bell & Howell, General Electric, Revere, and Telectro, to name some of the more popular companies of the day. Shure also made mics for many other lesser known companies, as well as the military. The majority of the mics made for other companies were made for reel to reel tape recorders, console units that had recording capabilities, and mobile or base station radio applications. Not all of these mics incorporated high impedance CR or CM elements. In fact, many had crystal and ceramic elements. The mics carried many different model #'s, way too many to list. The Shure models that came with a desk stand and PTT switch were known as "The "Dispatcher" (models 520SL & 520SLB)."